[1] Tech Univ Munich, Dept Phys, Walter Schottky Inst, D-85748 Garching, Germany
[2] Tech Univ Munich, Inst Adv Study, D-85748 Garching, Germany
[3] CALTECH, Inst Quantum Informat & Matter, Pasadena, CA 91125 USA
[4] Argonne Natl Lab, Div Mat Sci, 9700 S Cass Ave, Argonne, IL 60439 USA
[5] Harvard Univ, Dept Phys, Cambridge, MA 02138 USA